1. Technical Field
The invention relates to an optical head for scanning an optical record carrier having an information layer, comprising a radiation source for generating a radiation beam and an objective system for converging the radiation beam to a focus on the information layer, an actuator for displacing the objective system over a range in a direction transverse to its optical axis, an aberration detector for detecting an amount of an optical aberration in the radiation beam, an aberration compensator arranged in the optical path between the radiation source and the objective system, and a control circuit connected to an output of the aberration detector for controlling the aberration compensator. The invention also relates to a device for scanning an optical record carrier having an information layer and a transparent layer.
The information stored in an optical record carrier is arranged in tracks in the information layer of the record carrier. The information is written, read or erased by means of a focussed radiation beam that follows the track. The position of the focus is kept in the plane of the information layer by means of a focus servo that controls the axial position of the objective lens used for focussing the radiation beam. A second servo system controls the transverse position of the focus in order to keep the focus on the track being scanned. The transverse direction is the direction in the plane of the information layer perpendicular to the direction of the track. The second servo system causes the objective lens to move in the transverse direction, i.e. in a direction perpendicular to its optical axis, thereby moving the focus also in the transverse direction. On a disk-shaped record carrier the transverse direction corresponds to the radial direction; therefore the second servo system is also called the radial tracking servo.
2. Related Art
The trend of increasing information density on optical record carriers requires a commensurate decrease in the size of the focus of the radiation beam formed on the information layer. A smaller focus can be realized by increasing the numerical aperture of the radiation beam incident on the record carrier. However, an increase of the numerical aperture increases the susceptibility of the optical system in the head to optical aberrations. One of the aberrations is coma, caused by the transparent layer of the record carrier when it is not perpendicular to the principal ray of the radiation beam incident on the record carrier. Such non-perpendicular incidence of the radiation beam on the record carrier, generally referred to as tilt, may be caused by warping of the record carrier. Optical heads having a high numerical aperture require compensation of the coma caused by the tilt in order to scan the information layer of the record carrier properly.
European Patent Application No. 0 745 980 shows an optical head provided with a tilt compensator. The tilt compensator is an electrostriction device arranged in the optical path between the radiation source and the objective system. The known head is also provided with a tilt detector for detecting tilt of the record carrier. The output signal of the tilt detector is used to control the tilt compensator, which introduces a wave front aberration in the radiation beam that compensates the coma caused by the tilted record carrier. It is a disadvantage of the known device, that the aberration compensation does not operate properly when the optical head is following a track.
It is an object of the invention to provide an optical head that has a good aberration compensation independent of the tracking of the focus.
This object is achieved if, according to the invention, the control circuit is arranged such that the aberration compensator compensates the optical aberration to a degree less than the measured amount for substantially all displacements in the range. The invention is based on the insight that the aberration introduced by the aberration compensator causes other aberrations in the radiation beam when the objective system is displaced in the transverse direction with respect to the aberration compensator. The magnitude of the other aberrations depends both on the amount of the aberration introduced by the aberration compensator and on the displacement of the objective system. At increasing displacement the wavefront error of the other aberrations might become larger than the aberration to be compensated. In such a case the compensation of the aberration increases the wavefront error instead of decreasing it. The invention solves this problem by compensating only a part of the optical aberration instead of the entire measured amount of the aberration, as is customary in the known scanning devices. As a result, a small wavefront error will remain because of the uncompensated amount of the optical aberration to be compensated and a wavefront error due to the other aberrations caused by the displacement of the objective system, the latter wavefront error being smaller than in the known optical head because of the incomplete compensation. A compensation is called xe2x80x9cpartialxe2x80x9d if the compensation removes 90% or less of the RMS wavefront error to be compensated.
There are several ways to determine the degree of compensation. In a first preferred embodiment, the optical head is provided with an actuator for displacing the objective system in a direction transverse to its optical axis and the degree of compensation depends on a maximum value of the displacement of the objective system, the degree being independent of the instantaneous displacement. The total wavefront error now comprises two components: the first component due to the uncompensated aberration, which has a constant value independent of the displacement of the objective system, and a wavefront error due to the other aberrations, being zero for a centred objective system and reaching a maximum value when the objective system is at a maximum displacement. The degree of compensation must be chosen such that the wavefront error both with centred objective system and with objective system at maximum displacement are acceptable for the performance of the optical head. It is an advantage of this embodiment that the control of the aberration compensator can be relatively simple, in that the control circuit need have an input from the aberration detector only.
In a second preferred embodiment, the optical head comprises a position detector for determining a transverse position of the objective system, the control circuit being connected to an output of the position detector and to an output of the aberration detector, and the degree of compensation depends on the instantaneous displacement of the objective system. By using the position of the objective system, the degree of compensation can be optimized in dependence on the instantaneous displacement of the objective system, thereby further reducing the total wavefront error. In particular, the wavefront error for the centred objective system can be made substantially equal to zero.
In a preferred embodiment of the optical head, the aberration compensator reduces coma in the radiation beam, which may be caused by tilt of the record carrier. The aberration detector may be a tilt detector for measuring the tilt of the record carrier.
In another preferred embodiment of the optical head, the aberration compensator reduces spherical aberration in the radiation beam. The spherical aberration may be caused by thickness variations of a transparent layer on the record carrier through which the radiation beam is focused onto the information layer.
A further aspect of the invention relates to a device for scanning an optical record carrier having an information layer, comprising an optical head according to the invention, and an information processing using for error correction.